Lyme disease is the most prevalent arthropod-borne disease of humans in the United States and many other countries throughout Europe and Asia. The objectives of this project are to (1) use recombinant DNA techniques to express specific antigens of Borrelia burgdorferi to improve the serodiagnosis of Lyme disease, (2) characterize at the molecular level, isolates of the Lyme disease spirochete from a wide range of biological and geographical sources, and (3) examine adaptive molecular responses produced by B. burgdorferi during infection in ticks. During the last year, we directed most of our efforts towards the last objective. We examined the differential expression of outer surface proteins (Osps) by spirochetes in ticks. Spirochetes in the unfed nymphal ticks' midgut were examined by indirect immunofluorescence (IFA) for the presence of OspA or OspC. Only OspA was detected on spirochetes in these ticks that had not yet fed on blood. We examined antisera from mice infected with spirochetes by tick-bite or by inoculation of midguts from partially engorged, infected ticks. Antisera from all mice had strong antibody responses to OspC but not to OspA, indicating that B. burgdorferi produced OspC, but little or no OspA, during early infection in mammals. Spirochetes were grown at 24 degrees C and 37 degrees C and then examined by SDS-PAGE and immunoblot. Spirochetes continued to produce OspC when grown at 37 degrees C but very little at 24 degrees C, demonstrating that the synthesis of OspC was regulated, at least in part, by temperature. Ticks attached and feeding would experience both an increase in temperature and the influx of fresh blood into the midgut where spirochetes reside. Midguts from infected I. scapularis nymphs were examined immediately after the ticks had engorged in 3 days on normal mice. In contrast to the spirochetes in unfed ticks, spirochetes in all of the tick smears stained for OspC were positive, demonstrating that during tick feeding spirochetes in the midgut synthesized OspC. However, the incubation of infected, unfed ticks for up to 6 days did not stimulate the spirochetes to produce OspC. This change on the spirochete's surface in the feeding tick, controlled in part by temperature and blood, has relevance to both vaccine development and serodiagnosis of Lyme disease. To our knowledge, this is the first time that a tick-borne bacterial pathogen has been shown to alter its expression of a specific surface component during tick feeding.